A dual fuel engine can typically operate in two modes. In a strictly liquid fuel mode a liquid fuel, such as diesel fuel, is injected directly into an engine cylinder or a precombustion chamber as the sole source of energy during combustion. In a dual fuel mode a gaseous fuel, such as natural gas, is mixed with air in an intake port of a cylinder and a small amount of diesel fuel is injected into the cylinder or the precombustion chamber in order to ignite the mixture of air and gaseous fuel. In such dual fuel engines, one or more of such gaseous fuel admission valves are positioned between a gaseous fuel inlet region and an air intake region of the engine. The pressure of the gaseous fuel inlet region is regulated in attempt to maintain the pressure within such region at a predetermined set amount higher than the pressure within the air intake region so that when the valve is opened the gaseous fuel will pass into the air intake region for mixing with the air. Thus, there generally exists some differential pressure across the gaseous fuel admission valves.
A solenoid actuated gaseous fuel admission valve such as that described in U.S. Pat. No. 5,398,724 can be used to deliver the gaseous fuel in such engines. The nature of solenoid actuated gaseous fuel admission valves is such that a current delivered to a solenoid coil thereof actuates the valve. However, gaseous fuel admission valves have certain limitations, including that the valve cannot be properly actuated, that is opened, if the pressure difference across the valve exceeds a certain maximum level. The maximum differential pressure rating for a given valve may vary according to size and make of the valve.
During certain operating conditions within a dual fuel engine it is possible that the differential pressure across the gaseous fuel admission valves will exceed the maximum differential pressure of the valves, such that the engine cannot be properly operated in a dual fuel mode. For example, one such case would be where the engine is operating in a dual fuel mode at a high engine load. In such case the pressure withn the air intake region is high and the pressure within the gaseous fuel inlet region is regulated to a slightly higher level. Engine operation might then be switched to a liquid fuel mode. During the liquid fuel mode if the load on the engine drops, the pressure within the air intake region also drops. However, the pressure within the gaseous fuel inlet region remains high, and possibly high enough that if engine operation were to switch to a dual fuel mode at the low engine load, the differential pressure across the gaseous fuel admission valves would exceed the maximum differential pressure rating of the valves.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.